In general, an Ethernet OAM (Operation Administration and Maintenance) protocol based on the ITU-TY. 1731 and the IEEE 802.1ag has been used as a protocol for maintaining and monitoring networks. In the Ethernet OAM protocol, an MEP (Maintenance entity group End Point) and an MIP (Meg Intermediate Point) are defined as components. The MEP and the MIP are located in an end point and an intermediate point in a path (ME: Maintenance Entity), respectively, and monitor a communication network in a unit of path connecting the MEP and the MIP with each other. The ME is defined for each MEG (Maintenance Entity Group) level that represents a layer to be monitored and is set in a communication network such as a VLAN (Virtual Local Area Network).
As a typical network monitoring function of the Ethernet OAM protocol, a CC (Continuity Check) function has been used, for example. With this CC function, each MEP periodically transmits a CCM (Continuity Check Message) packet in a multicasting manner through an MIP to all opposite MEPs included in the same MEG. Accordingly, each of the MEPs may check continuity among the MEPs in the same MEG by monitoring reception of CCM packets transmitted from the other opposite MEPs. Since each of the MEPs periodically transmits CCM packets at an even time interval, when one of the MEPs does not receive CCM packets from the other opposite MEPs for a predetermined period of time (for example, 3.5 times a periodic transmission interval), it is determined that the continuity is not attained. By this, the MEP may detect network failure.